1. Field of the Invention
The invention relates to procedures and devices for fragmenting ions, preferably biomolecular ions in tandem mass spectrometers.
2. Background of the Invention
Over the last decade, mass spectrometry has played an increasingly important role in the identification and characterization of biochemical compounds in research laboratories and various industries. The speed, specificity, and sensitivity of mass spectrometry make spectrometers especially attractive for rapid identification and characterization of biochemical compounds. Mass spectrometric configurations are distinguished by the methods and techniques utilized for ionization and separation of the analyte molecules. The mass separation process can include techniques for ion isolation, subsequent molecular fragmentation, and mass analysis of the fragment ions. The pattern of fragmentation yields information about the structure of the analyte molecules introduced into the mass spectrometer. The technique of combining ion isolation, molecular fragmentation, and mass analysis is referred to in the art as tandem (or MS/MS) mass spectrometry. MS/MS mass spectrometry is typically implemented as tandem in space (such as triple quadrupole instruments) and tandem in time (such as 3D ion traps) instruments. In tandem in space instruments, precursor ions are selected at a given set of coordinates, and then transferred to different set of coordinates for ion dissociation, and the products are analyzed at yet another set of coordinates. MS/MS in time means that precursor ions are selected at given set of coordinates and then exposed at a later time to dissociation at the same coordinates, followed by yet a later episode of mass analysis at the same coordinates.
One common method for ionizing biomolecules and organic compounds is electrospray ionization (ESI) whereby ions are ionized at atmospheric pressure outside the mass spectrometer via charging, dispersing and evaporating of small droplets. These ions are introduced via atmospheric pressure interface into the vacuum of a mass spectrometer. Matrix assisted laser desorption ionization (MALDI) is another widely used method for ionization of larger biomolecules. In this technique, analytes are mixed with a matrix which absorbs laser irradiation and facilitates ionization. By using pulsed lasers for one-step desorption and ionization, MALDI has application under both reduced pressure and atmospheric pressure conditions.
Fragmentation of ions can be achieved in commercial tandem mass spectrometers through collisionally induced dissociation (CID) with buffer gas molecules in a quadrupole collision cell, as described, for example, in U.S. Pat. No. 6,285,027, the entire contents of which are incorporated herein by reference. In radio frequency ion traps (both three-dimensional and two-dimensional traps), ions can also be fragmented by collisions with a buffer gas. In ion traps, the oscillation of ions to be fragmented can be excited by a bipolar alternating field, as described by Louris, J. N. et al, Anal. Chem. 1987, 59: 1677, U.S. Pat. No. 6,177,666 and U.S. Pat. No. 5,420,425, the entire contents of which are incorporated herein by reference. In CID, the energy of collision is quickly redistributed over the large number of vibrational degrees of freedom available in large biomolecules. The energy redistribution causes preferable cleavage of week bonds. Thus, the CID method seldom provides sufficient MS/MS sequence information for proteins larger than 10 kDa. Since the excitation in CID is not specific, the most labile bonds are typically cleaved (which are often a modifying group) and not necessarily the structurally important bonds.
Electron capture dissociation (ECD) is a more recent fragmentation technique that utilizes an ion-electron recombination reaction, as described by Zubarev et al, J. Am. Chem. Soc. 1998, 120: 3265-3266, the entire contents of which are incorporated herein by reference. The maximum cross section for the ion-electron interaction is achieved at very low electron energies (e.g., lower than 0.5 eV) and exceeds the collision cross section for interacting with neutrals by about 100 times. To date, ECD has been implemented in ion cyclotron resonance Fourier transform mass spectrometers (ICR-FTMS) with electrons injected directly into ICR cell. Realizing ECD in other types of mass spectrometers has so far been unsuccessful due to the difficulties of introducing low energy electrons into strong and/or varying electrical fields present in ion traps, quadrupole ion guides and sector mass analyzers.
There is a demand for simple and effective ion fragmentation techniques.